is the normal way to boot from a tbxi-blessed file. If MOS-CD is inserted, OF will start from first tbxi-file which is found (that on the CD).
To boot from CD in OF: Perhaps your CD-Drive is jumpered wrong and is now known as CD1 by OF - try that.
If that won't work, type devalias
to see all mounted devices in OF and hopefully your CDs OF-name.
If not, you can adress the CD directly via device-tree, which is a bit more tricky......see MAC-OF-documentation.
1xPeg2, PowerMac G5, PowerbookG4, 2x MacMiniG4, Efika (again), A3000T, A3000D and life is never boring.....